1. Field of the Invention
This invention relates to a transducer providing a digital output signal which varies as a function of a parameter.
2. Description of the Prior Art
A capacitive sensor circuit wherein the capacitor value, representative of the sensed parameter, is sensed by a repeated transfer of quantities of known charge is shown in U.S. Pat. No. 4,039,940 issued Aug. 2, 1977. In this particular patent, the value of the capacitor is not represented in ditital form, however.
A prior art capacitive pressure transducer used to measure turbine compressor discharge pressure is described in a technical paper published by The American Society of Mechanical Engineers, Gas Turbine Division, United Engineering Center, 325 East 47th Street, New York, New York 10017, and entitled "A Pressure Transducer For An Electronic Field Control" their No. 73-GT-40. The publication teaches use of an oscillator for exciting a variable capacitor and a reference capacitor.
Additionally, a pressure transducer for a fuel control system employing a free running oscillator which impressed an AC voltage on a single variable capacitor was offered for sale by the assignee of the present application, more than one year ago. The operation of the free running oscillator of the device offered for sale was similar to that described in the ASME article.